A 2007 Mustang GT turbo kit range in price, starting from as little as $1,000 and reaching as high as $10,000 or more. The most common ones tend to be between $1,500 and $2,000.
With aftermarket turbochargers, what you pay for is very much what you get. Less expensive turbo kits will be noticeably inferior in quality and performance to the high-end ones. But any turbo should improve performance and fuel efficiencies.
Don’t forget that your GT comes with a turbocharger already installed by default. When planning your aftermarket modification, you’ll need to take that into consideration.